16 Prakt. Met. Sonderband 52 (2018)
stack. This definitely comes in handy for macrofractography, since this technique retains
valuable colour information from the fracture surface. Macrofractographic fracture features
can also be studied using SEM images at low magnification. Apart from the much higher
resolution an SEM offers over LOM, arguably the most valuable advantage of the electron
column is the much greater depth of field of up to 2 mm, compared with only some nm in a
an LOM. With an SEM, large fracture surfaces can be brought entirely into focus.
Among the macrofractographic fracture features, the tell-tale, half moon-shaped beach
marks are the most prominent and well-known. They offer evidence, often detectable with
the naked eye, that the failed component did indeed fracture by fatigue. In between the
beach marks and parallel to them, the lucky failure analyst might find microfractographic
fracture features that are called fatigue striations. They are the exclusive realm of the
SEM. Fatigue striations are very small ridges on the fracture surface, which are produced
in ductile materials by the propagating crack. There number and spacing might be
indicative of the fracture mode that eventually "killed" the part. Sometimes they even
provide some means to estimate the loading situation and the level of alternating stresses
that caused the failure. Fatigue striations could be as finely spaced as only some tens of
nm and as coarsely as some pm. Sometimes the number of beach marks as
macrofractographic fracture features can be related to individual loading change events,
Figs. 1-3.
mn) l#2][#] =) ws] ov Lins!
Root C
As ever
factogre
fo deck
ie
wel. Fe
or
Fig. 1: High Cycle Fatigue (HCF) Fracture of Combustion Chamber Casing ne
in a Heavy-duty Gas Turbine Engine. Material is GX23CrMoV12-1, 1.4931 ei
approximate location of muitipt
‘crack initiation sites
beach mark~
final forced ~ —— IE side)
rupture shear lip of final ~~
forced rupture
Fig. 2: HCF Fracture of Compressor Vane in a Heavy-duty Gas Turbine Engine.
Material is X20Cr13, 1.4021
20 mm